John Hartson played for Arsenal, West Ham and Celtic during his career. Photograph: Steve Welsh/PA

John Hartson, the former Wales international who is battling cancer, has been offered the support of one of former clubs, Celtic.

The 34-year-old has been diagnosed with testicular cancer which has now spread to his brain, according to the Scottish Sun. Hartson was given the news at the weekend following tests in Swansea after complaining of severe headaches.

Hartson was signed for Celtic by Martin O'Neill in a £6m deal in 2001 and was a popular figure with fans during the five years he spent in Glasgow. During that time, he scored 89 league goals and was voted player of the year by both his fellow professionals and the Scottish Football Writers Association.

"John Hartson is a man whom Celtic Football Club has immense respect and affection for," said a club spokesman. "He has served the club, and the game of football in general, with distinction over many years.

"Clearly, everyone at the club fully supports John and his family at this extremely difficult time. It is very difficult for us to understand what John is going through at the moment, but we will offer any support we can to a great Celtic player. The thoughts and prayers of the entire Celtic family are with John at this time."